Lambda trigger on stream events in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Lambda event record processing?

Given a DynamoDB stream event with the following record snippet, what will the Lambda function log for the NewImage attribute?

{
  "Records": [
    {
      "eventID": "1",
      "eventName": "INSERT",
      "dynamodb": {
        "NewImage": {
          "UserId": {"S": "123"},
          "Score": {"N": "42"}
        }
      }
    }
  ]
}

Assume the Lambda code logs event.Records[0].dynamodb.NewImage.UserId.S and event.Records[0].dynamodb.NewImage.Score.N.

A "UserId: 123, Score: 42"

B "UserId: S, Score: N"

C "UserId: UserId, Score: Score"

D "UserId: null, Score: null"

💡 Hint

Remember DynamoDB stream records store attribute values as objects with type keys like S for string and N for number.

The NewImage attribute contains the new item data with each attribute wrapped in an object indicating its type. Accessing UserId.S gives the string value "123" and Score.N gives the string "42".

🧠 Conceptual

intermediate

Which event types trigger a Lambda from DynamoDB Streams?

Which of the following DynamoDB stream event types can trigger a Lambda function?

A INSERT, MODIFY, REMOVE

B CREATE, UPDATE, DELETE

C ADD, REPLACE, DELETE

D PUT, PATCH, DELETE

💡 Hint

Think about the standard DynamoDB stream event names.

DynamoDB streams emit events named INSERT (new item added), MODIFY (item updated), and REMOVE (item deleted). These trigger Lambda functions.

📝 Syntax

advanced

Identify the syntax error in this Lambda handler for DynamoDB stream

Which option contains a syntax error in this Lambda function snippet that processes DynamoDB stream events?

exports.handler = async (event) => {
  for (const record of event.Records) {
    console.log('Event ID:', record.eventID);
    console.log('Event Name:', record.eventName);
    console.log('New Image:', record.dynamodb.NewImage);
  }
}

A Missing semicolon after console.log statements

B Arrow function missing parentheses around event parameter

C Incorrect use of >= instead of => in arrow function

D No syntax error; code is valid

💡 Hint

Check the arrow function syntax and console.log usage.

The arrow function syntax is correct with one parameter without parentheses. Semicolons are optional in JavaScript. The code is syntactically valid.

❓ optimization

advanced

Best practice to handle batch size in Lambda triggered by DynamoDB Streams

You want to optimize your Lambda function triggered by DynamoDB Streams to process records efficiently without timing out. Which approach is best?

A Set batch size to 1 to process each record individually

B Set a moderate batch size (e.g., 100) and process records asynchronously with error handling

C Set batch size to 1000 and process all records synchronously in one invocation

D Disable batch processing and poll the stream manually

💡 Hint

Consider balancing throughput and Lambda execution time.

Setting a moderate batch size and processing records asynchronously with error handling balances performance and reliability, avoiding timeouts and partial failures.

🔧 Debug

expert

Why does this Lambda triggered by DynamoDB Streams fail to process some records?

Consider this Lambda function snippet triggered by DynamoDB Streams:

exports.handler = async (event) => {
  event.Records.forEach(async (record) => {
    await processRecord(record);
  });
};

Why might some records not be processed before the Lambda finishes?

A Event object is missing Records array

B Lambda times out before processing starts

C Using forEach with async functions does not wait for promises to resolve

D processRecord function is not awaited properly inside forEach

💡 Hint

Think about how async functions behave inside forEach loops.

Array.forEach does not wait for async callbacks to finish, so the Lambda may exit before all processRecord promises resolve. Using a for...of loop with await or Promise.all fixes this.